Wellbores drilled to subterranean formations to produce oil or gas often become unusable before the formation is depleted of recoverable oil or gas. For example, equipment may become lodged in the wellbore above or in production zones, preventing access to the production zones with pumps, logging and workover equipment. Production from a wellbore also becomes impaired over time. Some of this impairment is due to fines migrating with the formation fluids toward the wellbore. Scale can also be deposited by formation water near the wellbore because temperatures and pressures can be lower near the wellbore, resulting in decreased solubility of some dissolved components.
When a borehole becomes unusable, either a new well can be drilled from the surface, or the existing borehole can be used to the extent it remains serviceable, and a new borehole can be sidetracked from the bottom of this seviceable section. Sidetracking is often preferred because drilling, casing and cementing the serviceable portion of the wellbore is avoided. Sidetracking involves milling through a steel casing and doing so in a matter that does not create a sudden change in direction, or dog leg, in the wellbore. This has generally been attempted by either milling out an entire section of the casing, and then drilling through the side of the open hole, or by drilling through the side of the casing with a mill bit that is guided by a wedge called a "whipstock."
Attempts to drill through the side of a casing are often unsuccessful because either a dog leg is created, making the sidetracked wellbore unusable, the mill never gets outside of the casing, or the milling of the casing is simply to slow. Whipstocks typically used for these operations are also expensive. U.S. Pat. No. 4,765,404 discloses a whipstock typical of those used. This whipstock is locked in place and held by a packer assembly below the whipstock. A substantial packer is required to keep the whipstock from rotating or moving downward when the casing is being milled. This packer adds substantially to the cost of the sidetracking operation because it must be left in the abandoned portion of the wellbore when the sidetracking operation is complete.
Casings are considerably more difficult to penetrate than in situ formations, therefore, a mill is necessary to bore through steel casings. Ideal conditions for milling require high RPM's and adjusted bottom hole weight to sustain the RPM's. Many mills, because of their contact surface area, grab and bind against the milling surface slowing the RPM's and creating abnormal torque and recoil to the drill string and drilling rig. To eliminate this it is necessary to lessen the bottom hole weight which reduces milling time, or begin with a smaller mill and make multiple trips to obtain desired results. U.S. Pat. No. 5,199,513 discloses sidetracking mills that exemplify these problems. These mills cut through the casing with a single cutting stage, maximizing the torque required and the heat generated at the point the milling is occurring.
Milling a complete section of the casing and then sidetracking through the casingless wellbore wall is a more reliable method to sidetrack, but milling of the entire section of casing is very time consuming. Further, the success rate of such operations is often low because of the difficulty of sidetracking out of the wellbore before casing is again encountered below the open borehole.
It is therefore an object of the present invention to provide a reliable and inexpensive method to provide a sidetrack wellbore. In another aspect, it is an object of the present invention to provide a mill for penetrating a casing within a wellbore wherein the mill rapidly penetrates the casing without requiring an excessive amount of torque, and wherein the casing can be milled in a single operation.